The increasing complexity of the electronics used in high technology industries, such as aerospace, has enabled control of most parameters to the finest possible resolution. Although this complexity of control is desirable for flight control computers and ground avoidance systems, as examples, for many applications this complexity, and the increased costs associated therewith, is not required. For these components, a low cost, simple solution is desirable which dispenses with the complex algorithms and custom integrated circuits, but which provides the benefits of weight, reliability, precision, and ease of use available through electronic control.
In order to better define the inventive contribution detailed hereinafter, there is now offered a review of the state of the art, beyond which the invention described in this specification will be distinguished.
Brushless DC motor control is one area into which a large effort has been directed. The advancements in closed loop control is evident in the patent to Glennon, et al., U.S. Pat. No. 4,608,527 which provides optimized efficiency of the motor by varying the phase advance of the drive with the change in load demand. The system described in Glennon, et al. utilizes a real current error signal to control the phase shift of the waveform provided to the motor windings which effectively corrects the real current error due to load and improves the motor efficiency. This shifted waveform is generated by a phase advance waveform generator which, through a multi-phase controlled power supply circuit, drives each winding of the motor.
Although this system provides much needed advancements in the art for many applications, the cost and complexity of such control may make it unattractive to many other applications which currently utilize hydraulic control and human interface, such as raising and lowering loading ramps, etc. A great majority of low cost approaches to these applications utilize a simple induction motor energized via a three-pole switch or three-pole contactor operated by a person or simple logic. The results achieved by these systems are approximate, i.e. jogging a mechanism to an approximate location or to a limit switch. These simple systems, however, suffer from inrush current problems and mechanism oscillations when subjected to fast accelerations.
The invention to be described hereinafter approaches the motor drive control in a uniquely different manner in that a single switch coupling and de-coupling the neutral of the windings is utilized to energize and control the motor. Unlike the prior art systems which strive to control the input waveform to the motor windings, the present invention, through the use of a rectifier bridge and a single switch, allows direct connection to the source voltage. Control is then providing by disassociating the neutral of the windings, coupling them to the bridge, and allowing the switch to control the current flow through the windings by opening and shorting the neutrals. In this way, by selecting the proper relationship AC currents, the motor is energized to drive the load. Additionally, by pulse width modulating (PWM) the switch at start up, the current is controlled and a soft start of the motor is thus achieved, eliminating the problems associated with the simple prior art systems. Further, proper phase selection and "inch-worming" of the PWM switch control allows a phase advance of the resulting stator flux to drive the motor at various speeds.
In addition to providing a simple motor drive for low cost, low complexity applications, the present invention, through its single switch neutral coupling modulation, also provides output voltage regulation for both permanent magnet generator based supplies and transformer rectifiers.
Typically, prior art systems for controlling output voltage have concentrated on the output of the device. As illustrated in a patent to Cook, et al., U.S. Pat. No. 4,866,591, for a Regulated Transformer Rectifier Unit, a second secondary winding is utilized through a switch to maintain a constant voltage to a varying load. Although this system provides superior output control over earlier systems, it may be to complex and costly for many applications. Additionally, the current carrying and interruption capacity of the switch is required to be much greater on the secondary side of the unit for aircraft applications.
The instant invention is directed to overcoming one or more of the above problems.